The overall objectives of this research effort have been to investigate relationships between cellular and humoral host responses as stimulated by allografted tissues. Our primary experimental models have included the vascularized cardiac allograft undergoing acute rejection in unmodified rats and the graft whose survival has been prolonged by immunological enhancement of the host. During the past year, we have also broadened indefinitely surviving cardiac allografts in rats treated with the new immunosuppressive agent, Cyclosporin A. Generally, our recent aims have been two-fold: first, to define migration kinetics of various lymphocyte populations in modified and unmodified heart graft recipients and second, to assess in vivo the differential function of these lymphocytes and macrophages within the acutely rejecting graft and in various host lymphoid compartments.